When Concordia Healthcare Corp. agreed to issue a 14 percent stake to a private equity firm, the Canadian drug manufacturer tucked into the deal several layers of protection from shareholder activists. The agreement in October bars the buyout firm, Cinven, from transferring or reselling its Concordia shares to a list of almost 60 activist firms, including those run by Bill Ackman, Carl Icahn, and Dan Loeb, according to a regulatory filing.